For a transmission line, there generally exist two types of fault, i.e. permanent fault and temporary fault. And for HV/EHV system, over 90% of the faults are single-phase faults, and over 80% of the single-phase faults are temporary faults. Auto-reclosing is an effective way to improve the stability and continuity of a power system. However, if a transmission line with a permanent fault is auto-reclosed, there may be two risks, the first risk is that the primary devices may be damaged, and the second risk is that the system stability may be threatened. So it is very important to distinguish temporary faults from permanent faults in a transmission line.
For a transmission line without shunt reactors, a mutual and coupling voltage method is generally used. This method is based on the measuring of the voltage on an opened phase conductor. For a temporary single-phase fault, when the fault phase of a transmission line is isolated, the isolated phase conductor still possesses a voltage to ground, i.e. capacitive coupling voltage and inductive voltage from the unbroken healthy phase conductors. But for a permanent fault, the measured voltage mainly includes an inductive voltage from the unbroken healthy phase conductors, while the coupling voltage is nearly zero.
Long EHV/UHV transmission lines are often equipped with shunt reactors to compensate the distributed capacitance of the lines. A small reactor equipped between the neutral point of Y-connected reactors and the earth can partially compensate the phase-phase coupling capacitance. As a result, the power frequency capacitive coupling voltage may be very low in the case of either a permanent fault or a temporary fault. In this case, the methods based on the capacitive coupling voltage fail to distinguish a temporary fault from a permanent fault.